docs(arch): extract AlknetEndpoint into alknet-endpoint (ADR-083 Am. 2026-07-15)

Amend ADR-083 with the crate-extraction decision: the endpoint moves
from alknet-core into a new crate alknet-endpoint, mirroring the
alknet-client extraction (ADR-089). The ADR's shape (new + builder
methods + public dispatch + run/shutdown) is unchanged; only the
location changes.

The extraction is structural pruning, not an inline refactor. The
endpoint is a leaf consumer of core's shared types (zero handler crates
import it; 124 import sites for the other core modules). Extracting it
lets core shed quinn/iroh/rcgen/rustls-acme — handler crates no longer
transitively link those. A pure worker (client-only) does not pull
alknet-endpoint at all. The dep graph is symmetric: alknet-core is the
shared types crate; alknet-endpoint and alknet-client are the
server-side and client-side establishment crates.

New spec: crates/endpoint/README.md (the canonical endpoint spec).
core/endpoint.md is deprecated to a stub. Cross-references updated
across 8 docs (README, overview, tls, hub, client, core README, ADR-083,
ADR-089 references). Architecture review passed (3 critical, 9 warnings
— all addressed).

docs/architecture/crates/client/README.md

@@ -426,7 +426,7 @@ alknet-hub (uses AlknetClient for outbound worker dials)
 ├── alknet-client (the dial — the hub's dial_worker closure calls it)
 ├── alknet-channels-call (ChannelClient — the take-over)
 ├── alknet-call (CallAdapter, Dispatcher)
-└── alknet-core (AlknetEndpoint)
+└── alknet-endpoint (AlknetEndpoint)
 
 alknet-worker (uses AlknetClient to dial a hub)
 ├── alknet-client (the dial)
@@ -533,7 +533,7 @@ See [open-questions.md](../../open-questions.md) for full details.
   `ChannelClient::from_connection` (the take-over the dial feeds)
 - [ADR-017](../../decisions/017-call-protocol-client-and-adapter-contract.md)
   — `CallClient::spawn_dispatch` (the take-over the dial feeds)
-- [`crates/core/endpoint.md`](../core/endpoint.md) — `AlknetEndpoint`
+- [`crates/endpoint/README.md`](../endpoint/README.md) — `AlknetEndpoint`
   (the server-side complement)
 - [`crates/tls/README.md`](../tls/README.md) — `TlsClientConfig`
 - [`crates/call/client-and-adapters.md`](../call/client-and-adapters.md)